1. Field of the Invention
The present invention relates to a semiconductor laser device which has a dielectric reflective film on a optical exit face.
2. Description of the Related Art
In a semiconductor laser, a dielectric film is generally formed on an end face of a resonator obtained by wafer cleavage. A reflectance controllable film having a desired reflectance can be formed by choosing arbitrarily sorts, thicknesses and numbers of layers for the dielectric film formed on the end face. For example, the lower reflectance of the front end face and the higher reflectance of the rear end face may produce the higher output power.
However, it is not always enough to lower the reflectance of the front end face, and it is necessary to select the reflectance according to application, i.e., characteristics on demand of the semiconductor laser.
In the semiconductor laser of high output type, for example, the reflectance of the optical exit front end face is approximately 3% to 15%. In case the reflectance of 7% is sought, controllability of the reflectance is needed within 6%±1%. Usually, the reflectance of the front end face which emits laser light in the semiconductor laser is controlled by the thickness and the refractive index of a single dielectric film such as Al2O3, SiO2, etc.
FIG. 23 is a structural drawing showing an example of a conventional semiconductor laser device. A laser chip comprises a semiconductor substrates 1, such as GaAs, an active layer 2, cladding layers 3 formed above and below the active layer 2, and electrodes 4 formed above and below the cladding layers 3.
A semiconductor laser device comprises the laser chip noted above, a low reflective film 8 formed on a front end face of the laser chip, and a high reflective film 9 formed on a rear end face of the laser chip.
In general, for the low reflective film 8 on the front end face, a single layer film having an optical thickness of integral multiple of λ/4±α using laser oscillation wavelength λ in vacuum is used, wherein α is a correction coefficient for controlling a desired reflectance.
The front end face of the semiconductor laser is likely to rise in temperature due to high density of laser light. Therefore, the low reflective film 8 is typically formed of an aluminium oxide film with a thickness of 3*λ/4±α so as to serve as a heat dispersing plate (heat spreader).
FIG. 24 is a graph showing an example of reflectance of the conventional low reflective film with the wavelength dependency. FIG. 25 is a graph showing an example of reflectance of the conventional low reflective film with the thickness dependency. Here, for the low reflective film 8, an aluminium oxide film with a thickness of 318.9 nm (α=+17 nm) is formed on the front end face of a red semiconductor laser with an oscillation wavelength λ=660 nm. Incidentally, the equivalent refractive index of the laser chip is 3.817.
Referring to the graph of FIG. 25, when the reflectance of the low reflective film 8 is controlled within 6%±1%, it is seen that the thickness of the low reflective film 8 must be controlled with an accuracy of ±1%, i.e., nearly ±3 nm with respect to the design value of thickness da =318.9 nm. Such an accuracy of thickness is difficult to obtain by means of vacuum evaporation or sputtering which is generally used for forming an optical thin film, resulting in a lower yield of manufacturing a semiconductor laser.
The related prior arts are listed as follows: Japanese Patent Unexamined Publication JP-2001-77456-A (2001), and Japanese Patent Publication JP-3080312-B (2000).
However, in case of attaining the low reflective film 8 having a reflectance within 6%±1%, for example, variation in thickness of the single layer film of aluminium oxide must be limited within ±1%, resulting in the stricter controllability of reflectance and lower yield of manufacture.